Currently, a charge-coupled device (CCD) linear sensor and a CCD image sensor are the two most popular solid state imaging devices. Generally, in a conventional CCD sensor of an interline-transfer type, a plurality of photo diodes, as light receiving elements, are arranged horizontally and vertically; a vertical scanning CCD shift register is prepared adjacent to and parallel with a column of corresponding vertically arranged photo diodes; and a serial output from the vertical scanning CCD shift register is inputted to a CCD in a horizontal scanning CCD shift register.
Charges, i.e., photo-charges generated and accumulated in a process of photoelectric conversion by employing a photodiode, are transmitted to a corresponding adjacent vertical scanning CCD en bloc in response to a preset clock signal; and then the charges are progressively shift registered in a corresponding vertical scanning CCD shift register by a preset sequential clock signal. A serial charge output from the corresponding vertical scanning CCD shift register is fed to a horizontal scanning CCD shift register. The charges are progressively shift registered in the corresponding horizontal scanning CCD shift register in response to a preset sequential clock signal to thereby output a time series analog signal.
In this conventional solid state imaging apparatus, e.g., a digital camera, employing such a solid state imaging device, there is prepared an electric or mechanical shutter for controlling an input of incident light to the solid state imaging device, wherein only during the period while the shutter is opened, the incident light is irradiated on photodiodes in the solid state imaging device to thereby generate and accumulate charges. The incident light may include natural sunlight, light originated from a fluorescent or electric lamp and light reflected from a subject. The incident light may further include light reflected from a supplementary light source such as the one used in a stroboscope.
As is well known in the art, if the intensity of background light, e.g., due to natural sunlight, is too high, there may arise the so-called backlight phenomenon. In such event, an image for a subject becomes dark and unclear; and a boundary between a subject image and a background image becomes vague. In some cases, it is required that background image for a target subject be eliminated to obtain a clear image for the target subject.
An undesired background image for the target subject can be eliminated through an image processing procedure by employing a computer. However, if the boundary between an image of the target subject and a background image thereof is unclear, it is difficult to eliminate the background image properly.